Air hardening bearing steel and bearings made therefrom



United States Patent M 3,298,827 AIR HARDENING BEARING STEEL AND BEARINGS MADE THEREFROM Chester F. Jatczak, Canton, Ohio, assignor to The Timken Raider Bearing Company, Canton, Ohio, a corporation of Ohio No Drawing. Filed Sept. 13, 1963, Ser. No. 308,648

4 Claims. (Cl. 75-128) All steel bearings, at least those made in this country, are hardened, as far as I am aware, by liquid quenching, chiefly in oil, from an austenitizing temperature appropriate for the steel composition being used, Thus, oil quenching is used for bearing components made from through hardening steels such as 52100;and also for those made from case carburized compositions such as 4620, 8620, 4118, etc.

Such liquid quenching causes thebearing components to distort, warp and change shape to varying degrees due to the severe cooling action of the oil or other liquid quenching medium. For that reason'present practice is to use quenching plugs or restricting fixtures, the purpose of which is to physically and mechanically limit or hold distortion within tolerable grinding limits. The manufacture and maintenance of such quenching plugs may be, and often is, the major cost incurred in the manufacture of such bearing components as bearing races. That is, the cost of these quenching plugs commonly will make the production of hearings in small lots, and particularly the large diameter ones, for instance in excess of 3 feet in which distortion is a major problem, uneconomical.

I have found, as described fully hereinafter, thatby utilizing slower cooling procedures, such as air cooling, it is possible to maintain quenching distortion within tolerable limits and thus to alleviate the use of quenching plugs altogether. The success of the air cooling procedure rested, however, on my development in accordance with this invention of a steel which would harden satisfactorily throughout large bearing sections. 1

Of the standard bearing steels, S2100 cannot be through air hardened satisfactorily in sections larger than about A-inch round while the carburizing grades are limited to about a /2-inch round for air hardening. Prior to this invention no steels suitable for hearing manufacture were available for through hardening to high hardness levels in heavier sections by cooling in air.

It is among the objects of this invention to provide Q low alloy carbon steels which may be through hardened to Rc 60 by cooling in still air in sections up to 6 inches, and particularly such steels that may be air hardened from a low hardening range of about 1550 to. 1600 F. while undergoing distortion to an unobjectionable extent.

Another object is to provide such steels which when hardened and tempered undergo dimensional changes over the green size that are extremely small and greatly less than are encountered with known bearing steels hardened by liquid quenching whereby the need for quenching .plugs becomes unnecessary, and finish grinding is reduced to a minimum.

Still another object is to provide steels in accordance with the foregoing objects which when tempered at 300 to 400 F. produce satisfactory hardnesses of 58 to 60 Rc in sections up to at least 5 inches, and which may be annealed to a readily machinable hardness by the use of reasonable production cycles.

Yet another object is to provide steels in accordance with the foregoing objects that possess good forging characteristics and which may be pierced satisfactorily.

A further object is to provide steels for through hardening in heavy sections by air quenching which possess exceptional impact strength in unnotched specimens, and which are adapted for use as cold work 1001 and die steels 3,298,827 Patented Jan. 17, 1967 ICC where toughness or resistance to breakage is of primary concern.

A still further object is to provide wrought, i.e., rolled, forged and pierced articles embodying the properties of the foregoing objects.

Other objects will be understood from the following specification.

The invention is predicated upon my discovery that the foregoing objects and other advantages are attained with steels containing from about 0.70 to 0.80 percent of carbon, about 0.90 to 1.20 percent of chromium, about 1.05 to 1.35 percent of magnanese, about 1.20 to 1.40 percent ofm'olybdenum, and about 1.30 to 1.65 percent of nickel, 'with' silicon held at about 0.20 to 0.35 percent. Within the foregoing ranges there is used in the preferred practice of'the invention a steel containing about 0.75 percent of carbon, about 1.05 percent of chromium, about 1.20 percent of manganese, about 1.30 percent of molybdenum, about 1.50 percent of nickel and about 0.30 percent of silicon.

As exemplifying the advantageous properties of my new steels which render them outstandingly useful for the manufacture of large bearings byair hardening, reference will now be made to actual data obtained with a steel in accordance with the preferred embodiment analyzing 0.74 percent of carbon, 1.08 percent of chromium, 1.18 per cent of manganese, 1.31 percent of molybdenum, 1.53 percent of nickel, and 0.33 percent of silicon. This heat was made by induction furnace melting in 30-pound quantities which were then forged to l A-inch squares at 2050 F., during which no forging difficulties were encountered.

Annealing specimens were cut from the forged bars then treated using the following annealing cycles:

Cycle AHeat to 1320" F., equalize, heat 10./hour to 1500" F., hold 2 hours, cool 20/hour to 1000 F. BHN=235 v Cycle B'-Heat'to 1320 F., equalize, heat IOf/hour to 1460 F., hold 2 hours, cool 10/hour to 1100 F. BHN=217 Cycle CSame as Cycle 13, except cool l0 /hour to A hardness of 217 Brinell is acceptable for the machining of bearingraces, and it will be observed that this hardness was met or bettered by cycles B to E, incl.

Hardening or austenitizing characteristics were likewise surveyed on the forgedbars by heating annealed specimens l A-inch square by %-'inch thick to temperatures from 1400 to 1700 F. in 50 F. intervals, cooling in still air and measuring the resulting Rockwell C hardnesses. The results are given in Table I:

TABLE I.-SPECIMENS AIR HARDENED FROM TEM- PERATURE STATED, HARDNESS Rc 1500 F. 63.8 l550 F. 63.8 1600 F. 63.7

The as-hardened microstructures were studied also with reference to solution of carbides, grain size and the amount of retained austenite for each condition. Speci- TABLE II.-.SPECIMENS HARDENED IN AIR AT 15501600 F.

Treatment, F.: Rc As quenched 64.0 300 61.0 400 58.2

The air hardenability of the steel was then measured.-

Annealed l fit-inch square bars were turned to the standard Timken hardenability test specimen (1" diameter), and they were then austenitized within a 6-inch round by 12-inch long bar having a 1" bore at hardening temperatures from 1500 to 1750 F. Half of the test bar was" exposed outside of the 6-inch round at its center or core.

On air cooling of this mass the 6-inch round restricts the cooling of the 1-inch round test bar sufficiently to permit the tested steel to undergo air cooling conditions ranging from those of a I la-inch round to that of the 6-inch round itself. The data showed that when cooled in still air from 1550 F. a hardness of Re 60 is obtained at the center of a 3 /2 inch round of the steel of this invention. On the other hand, when cooled in still air from 1600 F. or 1650 F. a hardness of Re 60 can be obtained at the center of rounds of more than 6 inch section.

The exceptionally small changes in size or dimensions which occur during heat treatment of the steels of this invention may be exemplified by one series of tests. in which test specimens of the above heat %-inch round by 2.50-inoh were hardened in still air from 1550 F. and given successive tempering treatments. The data are given in the following table: V

TABLE IV.UNNOTCHED IMPACT STRENGTH OF STEEL OF THIS INVENTION AIR HARDENED FROM 1550 F.

Heat X109 (S.T.) 1 Heat X109 (D.T.) 2

Ft.-Lbs. Rockwell FL-Llos. Rockwell ucn As Air Quenched 26. O 04. 5 At 300 F 75.0 62.0 94. 61. 0 At 350 F 77. 0 60. O At 400 F. 85.0 59. 77. 0 59. 0 At 500 F 89. 0 58. 0 90. 0 58.0 At 600 F 89.0 57.0 96. 0 55. 5 At 900 F 120. 0+ 51.0

1 Single temper. Double temper.

The following Table V compares the unnotched Izod impact strength of the steel of this invention with comparative impact data on 52100 steel, commonly used for bearings an oil hardened. The steel of this invention shows far better impact strength at equivalent tempering temperatures and hardnesses than 52100. In fact, the impact, properties of the steels of this invention render them especially suited for hearing applications but in addition the unnotched impact properties are such as to adapt the steels admirably also for cold work die and tool steels as for heavy duty cold shears where toughness and resistance to breakage are of primary concern.

TABLE V,-UNNOTCIIED IZOD TESTS "'6" sif 96.0 55.5

l Qucnehed from 1,550 F. in air, double tempered. Quenched from 1,550 F. in air, single tempered.

Actually, my new steels exhibit the highest impact strength'known tome for through hardening bearing compositions as Well as cold work steels.

TABLE r11 Tempering Original Size Change. in Size Change Size Change Total Change Temperature Green After vHardening, After 1st" Hardening To After 2d 1st to 2d Green to 2d Size Hardening In./In.' Temper Temper, 1n./In. Temper Temper Temper, In./In..

As is evident, bearings made of these steels would require only minor stock allowance for size change which is a great advantage since the need for quenching plugs is completely eliminated. Thus when hardened in still air and tempered to 58 to 60 Rc the dimensional changes average from nil to 0.0003 inches per inch over the green size, which is most unusual in air hardening bearing'steels.

Additionally, the impact properties were determined in both the notched and the unnotched conditions on 0.394- inch Izod specimens in oversize form, 0.440-inch square, then finish ground to 0.394-inch for testing. In steels of the carbon range of the present ones the unnotched Izod test is the proper one for impact study. The specimens were cooled in still air from 1550 F. and given a double temper at the temperatures shown in the following tabulation. The unusually high impact strength at the high hardnesses characteristic of the present invention are evident from Table IV.

The levels of austenite which may be retained in the new steels were determined by hardening in still air from 1550 F. and tempering at various temperatures as shown in the following table together with the consequent retained austenite:

TABLE VI.AUSTENITE RETAINED AFTER IIA RDENING IN STILL AIR FROM 1,550 F. AND TEMPERING AS SHOWN Normally for bearing purposes steels would be tempered between 350 to 400 F. to a minimum hardness of 58 Re. Under these conditions about 18 to 20 percent of austenite would be retained, Which is about the level presently found in some hardened and tempered bearing race surfaces. As the tabulation shows, only a single temper is necessary for this purpose.

The new steels hardened at temperatures ranging from 1500" F. to 1800 F. showed that the grain size ranged from 9.5 to 7.0, that is, the steels remain very fine grained over a wide hardening range, and this survey explains the excellent impact properties or toughness of these steels.

The critical table temperatures of the steel were determined to be as follows:

TABLE VII.-AUSTENITIZED 1 55O F. FOR 30 MINUTES Ac 1275 F., Ar 1225 F. Ac 1450 F., Ar 1290 F. Ms3 85 F.*

*Established by isothermal holding at temperature for 5 hours.

In the practice of the invention ingot cooling may be hot run or Warmer cool, whichever is more convenient, For forging or rolling the ingot or reforged billet should be equalized at 1300 F. to 1400 F. before heating to forging temperature, and the ingot or billet should then be forged at 2100" to 2200 F. Without cooling below 1700 F. At the conclusion of rolling or forging the worked piece should be cooled in sand or other slow cooling medium.

To soften billets for conditioning, such asgrinding, it is preferred to temper at 1225 F. for 4 to 6 hours although alternatively this may be accomplished by either of the following annealing treatments: (1) Heat to 1500 F., equalize, furnace cool to 1280 F. followed by cooling at 10 F. per hour to 1100 F., then air cool. This is normally productive of a maximum hardness of 223 brinell. (2) The annealing cycle just given will result in a spheroidized structure. Lamellar structures cut more easily and efiiciently and to produce a structure suited to sawing, the following alternative annealing cycle may be used for the cold sawing of etch slabs or bloom or billet multiple: heat to 1650 F., equalize, furnace cool to 1280 to 1300 F., cool at 10 F. per hour to 1100 F., discharge. A hardness of about 269 brinell will result. At this hardness level the lamellar structure provides a 100% improvement in sawability over the spheroidized structure, of about 223 brinell. This holds true for tests on circular cold saws, power hack saws and band saws.

As further evidencing the versatility and utility of the new steels it is to be observed that production heats were pierced for the production of seamless tubing, and this was accomplished easily and without unusual difiiculties.

Hardening practice desirably in values: for rounds and squares up to 2 inches and flat plates to 1.5 inches thick, heat to 1550 F., 0001 in air; rounds and squares 2 to 5 inches, flats and plates 1.5 to 3.5 inches thick, heat to 1600 F., cool in air; rounds and squares above 5 inches,

6 flats and plates 3.5 to 4.5 inches thick, heat to 1650 F., cool in air. Allow one hour per inch of thickness in heating time to insure hardening temperature is attained. Rounds and squares larger than 6.5 inches, and flats and plates above 4.5 inches will not through harden in air to over Re 60.

As to tempering, heat 2 hours at 300 F. for R0 to 62; at 400 F. for Rc 58 to 60; and for bearing races at Rc 58 minimum temper at 350 F., allowing one hour per inch of thickness to reach temperature.

I consider that the balanced compositions of this invention are critical in achieving the unusual combination of properties described and exemplified above.

According to the provisions of the patent statutes, I have explained the principle of my invention and have described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

I claim:

1. An air hardening steel consisting essentially of about 0.7 to 0.8 percent of carbon, about 0.9 to 1.2 percent of chromium, about 1.10 to 1.35 percent of manganese, about 1.2 to 1.4 percent of molybdenum, about 1.3 to 1.65 percent of nickel, and about 0.2 to 0.35 percent of silicon balance iron, and characterized in sections up to 5-inch diameter of hardening to Rockwell C of 60 or higher upon air cooling from 1500 to 1650 F. followed by tempering at 350 to 400 F.

2. An air hardening steel consisting essentially of about 0.75 percent of carbon, about 1.1 percent of chromium, about 1.2 percent of manganese, about 1.3 percent molybdenum, about 1.5 per-cent of nickel, and about 0.3 percent of silicon balance iron.

3. A wrought and heat treated steel article of the composition of claim 1 characterized by a Rockwell C hardness of about 60 in a section up to about 5 inches thick.

4. A seamless steel tube of the composition of claim 1.

References Cited by the Examiner UNITED STATES PATENTS 1,464,174 8/1923 Finkl 148-36 1,558,918 10/1925 Peters 128 1,660,790 2/1928 Herman 75128 2,331,900 10/1943 Finkl 75-128 3,144,362 8/1964 Bradd 148-36 FOREIGN PATENTS 344,822 3 1931 Great Britain.

344,893 3/1931 Great Britain.

350,752 6/1931 Great Britain.

572,544 1/1958 Italy.

DAVID L. RECK, Primary Examiner.

HYLAND BIZOT, Examiner.

P. WEINSTEIN, Assistant Examiner. 

1. AN AIR HARDENING STEEL CONSISTING ESSENTIALLY OF ABOUT 0.7 TO 0.8 PERCENT OF CARBON, ABOUT 0.9 TO 1.2 PERCENT OF CHROMIUM, ABOUT 1.10 TO 1.35 PERCENT OF MANGANESE, ABOUT 1.2 TO 1.4 PERCENT OF MOLYBDENUM, ABOUT 1.3 TO 1.65 PERCENT OF NICKEL, AND ABOUT 0.2 TO 0.35 PERCENT OF SILICON BALANCE IRON, AND CHARACTERIZED IN SECTIONS UP TO 5-INCH DIAMETER OF HARDENING TO ROCKWELL C OF 60 OR HIGHER UPON AIR COOLING FROM 1500* TO 1650*F. FOLLOWED BY TEMPERING AT 350* TO 400*F. 